Hi While im not a postoffice broadband user im pretty sure it does not come with usenet access. So your going to have to sign up to a 3rd party usenet provider.
I can recommend these two providers that i have used personaly and found to be very good.
www.usenet-news.net Very reliable decent retention and prices.
or
http://www.news.astraweb.com/ also reliable ok prices and retention you won go wrong with either of these providers.
Blackmesa8